Dissolved Oxygen in Beer: How It Compares to Total Package Oxygen

When it comes to questions about oxygen in beer, I think the one I’m asked most often is, “What is the difference between dissolved oxygen and total package oxygen (TPO)?”  The main source of this confusion is that when measuring O2 in packages, the O2 in the headspace is often overlooked. If you don’t take headspace oxygen into account, then you are measuring a partial concentration, period. So let’s talk about the differences and what each one tells you.

A significant number of craft brewers have a dissolved oxygen (dO2) analyzer they use to measure the dO2 content of their beer in process. The most common point of measurement is the finished beer tank. The beer in a finishing tank will have O2 pickup from the empty vessel and from the filtration process, plus it will pickup more O2 as it goes through packaging.

Once the beer is packaged, however (assuming good packaging,) rapid O2 pickup from outside sources all but stops. So what can we tell about how much oxygen actually made it into the package?  It is not a simple matter of measuring the O2 in the beer.  The package must be shaken to equilibrate the oxygen in the beer and the headspace before the 02 in the beer is measured, and that number must then be used to calculate your TPO. Let’s think about what it is possible to measure and what each thing tells you.

Package dO2 –

The easiest measurement to take on packaged beer is the dO2 of a package just off the filler without shaking the beer. It is important to measure as quickly as possible, so the product does not “consume” the oxygen in the beer. (Residual or live yeast may be hungry, plus oxidation by trace metals, etc.) In some packages there is a measurable difference within five minutes and in other packages the rate of oxygen consumption takes significantly longer, sometimes hours. It is always best to measure as quickly as possible.

This unshaken package measurement represents the combination of the dO2 of the beer at the base of the filler and the oxygen pickup of the filler. Oxygen picked up at the filler can be quite variable. Most fillers run at about 25 to 50 percent deviation, but in some cases it can be up to 100 percent deviation. The best way to measure the percent deviation is to determine the dO2 at the base of the filler and then measure six to ten packages and determine the variation of each package as compared to the average of all the containers. But remember: this measurement only tells you what is in the liquid. When measuring unshaken packages, any gas in the headspace is left uncounted.

Shaken Package dO2 –

When you shake a package of beer so that the partial pressure of the oxygen in the liquid is equal to the partial pressure in the headspace, it changes the characteristics of the oxygen partitioning in the package. If most of the oxygen in the package is locked in the liquid, then shaking the container will move the O­2 from the liquid to the headspace until equilibrium is reached.

So, you have measured the dO2 and then shaken the package. Now what do you do with the data? If you really want to quantify the TPO of the package you have to take into account the headspace oxygen. To do this accurately you need to know the headspace volume and the package temperature.

Total Package Oxygen –

When using the dissolved oxygen measurement, the TPO can only be calculated from a shaken package. To do this calculation you also need to know the headspace volume, liquid volume and the package temperature. The temperature and the headspace volume are critical values and small inaccuracies can alter the results significantly, but the liquid volume may be estimated by using the average fill volume. Once you have your figures, then you can use a TPO calculator to determine the concentration from your initial DO2 measurements.

My final thought is to not skimp on how much you shake the packages. Cold containers should be shaken for five minutes and room temperature cans or bottles need about three minutes. If you’d like a copy of a TPO calculator built into an Excel spreadsheet, then please click here to request one.

 

 

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How the Purity of Sparged Carbon Dioxide Affects the Oxygen Concentration of Beer

 

My last blog post discussed the importance of carbon dioxide purity when using injected CO2 to increase the CO2 concentration of your beer. The thing we learned is that the purity of CO2 must be very high (99.99% or better) when using injection, or you will at the same time significantly increase your dissolved oxygen levels. However, injection isn’t the only method for adding CO2 to beer. Sparging, in which CO2 is bubbled through beer (usually in a tank with slight over-pressure) is another common practice for boosting CO2. So in this post we will explore how sparging a finished beer tank with carbon dioxide impacts the final oxygen concentration of your product.

First, let’s do a quick review of what happens when you inject CO2. When you inject gas (usually into a pipe,) you are forcing a given weight of gas into the liquid under pressure. All of the carbon dioxide, plus any trace oxygen and nitrogen, gets pushed into the beer and dissolves completely, allowing you to calculate the weight of gas used and extrapolate from there your various gas concentrations.

On the other hand, when you sparge gas into a liquid the dissolved concentration of gas will be bound by Henry’s Law. Henry’s Law tells us that the amount of gas that will dissolve in a liquid will be proportional – at a constant temperature – to the partial pressure of gas in equilibrium with the liquid.  This means that the gasses dissolved in your beer will never be more concentrated that the partial pressure of the gas you are using to sparge.

The consequence is that, with any given CO2 concentration outcome desired, you will have significantly lower oxygen concentrations for sparged beer then for injected beer. For example, in injected beer the oxygen pickup from injecting one volume of 99.95% CO2 (at 0oC) into the beer when the oxygen concentration is 0.01% is 143 ppb. But a theoretical sparging of that same CO2 into the beer at atmospheric pressure would follow Henry’s Law, and your oxygen pickup would be about 7 ppb.

In real brewing situations, however, most brewers use tank overpressure to help get sparged CO2 into solution, so you would probably be picking up about 2 times the above amount, or 14 ppb. The table below shows the expected oxygen pickup given varied percentages of O2 traces (in your CO2) when measured at sea level and at 0oC:

Sparged CO2 at 1 V/V

0.001% O2

0.005% O2

0.01% O2

<1

3

7

My final thought is that CO2 purity isn’t nearly as important if you are sparging rather than injecting, since the amount of gas that will dissolve into your liquid is much lower. This also applies to the purity of the gas you use to flush air from tanks before filling.

Package O2 Measurements – What Can You Learn From Unshaken Containers?

 

When using a portable dissolved oxygen analyzer to measure package oxygen concentrations, you have two options:

  • Measure the package directly off the filler.
  • Shake the package until the liquid and headspace gases reach equilibrium and then measure.

Let’s dive deeply into interpreting the results of unshaken packages and learn what it tells you.

Since we aren’t doing anything to the container to equilibrate the headspace gas with the dissolved gas in the liquid, an unshaken package gives us a snapshot of these three oxygen influences:

  • Dissolved gas in the liquid right as it enters a filler.
  • Dissolved gas pickup during filling due to air in the package that has not been cleared before the package is filled.
  • Fill bowl O2 pickup in rotary fillers

The differentiation of these is easy to quantify. One is the dO2 at the base of the filler and the other is the dO2 measured in the package minus the dO2 at he base of the filler. Here’s an example:

  • dO2 of an unshaken package = 63 ppb or 0.063 ppm.
  • Base of filler dO2 = 18 ppb or 0.018 ppm.
  • Filler dO2 pickup = 45 ppb or 0.045 ppm.

Since it is relatively easy to measure just before the filler and just after filling, let’s discuss what influences the results of each.

The dO2 concentration of beer at the base of the filler is usually easy to control and is based on just a few potential influences. High values can be caused by:

  • High residual in the finished beer tank.
  • Oxygen pickup from a pump between tank and filler.
  • O2 pickup from a valve or fitting between tank and filler.

Likewise, if you make it a practice to regularly measure the dO2 at the base of your filler and then calculate the filler valve pickup, it can give you great feedback on when to service your filler. If the O2 at the base of the filler is low, but the unshaken dissolved O2 is high, then perhaps there are ways to alter your filler system to achieve lower values. Here are some potential areas of oxygen pickup:

  • Purging on rotary bottle fillers as impacted by vacuum pumps, CO2 purge duration, fill tube lengths, filler speed, and fill bowl characteristics.
  • Effectiveness of CO2 purge pressure and flow on an inline batch bottle filler.
  • CO2 purge time, fill tube lengths, filler speed, and fill bowl characteristics on rotary can fillers.
  • CO2 purge pressure and flow on an inline can filler.

So we can learn a lot from measuring gas content in unshaken packages, although it’s important to remember that an unshaken package won’t tell you if you’re picking up oxygen from a can-seamer or while fobbing bottles, since they can contribute to oxygen in your headspace.  Total Package Oxygen (TPO) takes into account headspace oxygen and is the only calculation that can give you a complete picture of the oxygen in your finished product.

My final thought is that understanding unshaken package dO2 measurements will help you troubleshoot some sources of package oxygen contamination. Next time we’ll examine shaken package measurements and tie it all back total package oxygen.

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